The present invention relates generally to the continuous separation of solid particles, such as soot, from a fluid, such as oil, by the use of a centrifugal field. More particularly the present invention relates to the use of a cone (disk) stack centrifuge configuration within a centrifuge assembly which includes a turbine wheel for rotatably driving a rotor. The turbine wheel is driven by jet nozzles tangentially aligned with the runner circular centerline.
Diesel engines are designed with relatively sophisticated air and fuel filters (cleaners) in an effort to keep dirt and debris out of the engine. Even with these air and fuel cleaners, dirt and debris, including engine-generated wear debris, will find a way into the lubricating oil of the engine. The result is wear on critical engine components and if this condition is left unsolved or not remedied, engine failure. For this reason, many engines are designed with full flow oil filters that continually clean the oil as it circulates between the lubricant sump and engine parts.
There are a number of design constraints and considerations for such full flow filters and typically these constraints mean that such filters can only remove those dirt particles that are in the range of 10 microns or larger. While removal of particles of this size may prevent a catastrophic failure, harmful wear will still be caused by smaller particles of dirt that get into and remain in the oil. In order to try and address the concern over small particles, designers have gone to bypass filtering systems which filter a predetermined percentage of the total oil flow. The combination of a full flow filter in conjunction with a bypass filter reduces engine wear to an acceptable level, but not to the desired level. Since bypass filters may be able to trap particles less than approximately 10 microns, the combination of a full flow filter and bypass filter offers a substantial improvement over the use of only a full flow filter.
While centrifuge cleaners can be configured in a variety of ways as represented by the earlier designs of others, one product which is representative of part of the early design evolution is the Spinner II.RTM. oil cleaning centrifuge made by Glacier Metal Company Ltd., of Somerset, Ilminister, United Kingdom, and offered by T. F. Hudgins, Incorporated, of Houston, Tex. Various advances and improvements to the Spinner II.RTM. product are represented by U.S. Pat. No. 5,575,912 issued Nov. 19, 1996 to Herman and by U.S. Pat. No. 5,637,217 issued Jun. 10, 1997 to Herman and these two patents are expressly incorporated by reference herein for their entire disclosures.
There is currently an engine operation phenomenon taking place which creates unacceptable levels of lube-oil soot. A majority of this lube-oil soot needs to be removed from the circulating oil due to the abrasive nature of the soot and the corresponding risk of unacceptable wear on critical engine surfaces and at critical engine interfaces. Increasingly stringent NO.sub.x emissions regulations are causing widespread usage of retarded injection and in some cases exhaust gas recirculation or water injection to further retard the combustion event. In turn, this reduces peak temperatures and causes NO.sub.x formation. However, delayed combustion allows soot deposition on exposed cylinder walls and subsequent transfer to the lube oil by the scraping of the rings. Engine data derived to examine lube-oil soot has revealed levels as high as seven percent (7%) in 250 hours of operation. While this lube-oil soot has a relative diminutive size on the order of 0.02 to 0.06 microns, it is still abrasive in nature and capable of causing wear at critical high pressure/load interfaces such as those found in valve train components. For additional information regarding the abrasive nature and wear, refer to SAE Paper No. 971631.
Of importance with regard to the present invention is the realization that removal of the extremely small soot particles by way of conventional filtration or by means of conventional centrifugal separators, including cone-stack designs, has generally proven to be fruitless. One of the limiting factors is the rotational speed that centrifugal separators are typically driven at. The typical or normal rotational speed for Hero-turbine centrifugal separators is in the range of approximately 5000 RPMs for a rotor with a 4.75 inch outside diameter cone stack and approximately 7000 RPMs for a rotor with a 3.50 inch outside diameter cone stack. These speeds are not fast enough to remove the soot at an adequate rate in order to control soot build up in the oil. Rates of approximately twice those listed are needed to effectively attack the soot build-up problem.
The oil in the sump begins as clean oil and, over time with operation of the engine, soot gradually builds up. The objective is to control the percentage of soot in the sump oil. While an equilibrium condition will, in time, be established where the removal rate is the same as the add rate, the key is the percentage of soot. The governing equation is the following: ##EQU1## The removal efficiency and the flow rate are coupled such that just doubling the flow rate cuts the efficiency by one-half. The key is the removal efficiency. If this can be increased, the soot concentration in the sump will be decreased without altering any other factors or components.
In view of the discussed concerns and issues with regard to present centrifugal separator designs, it would be an improvement to devise a configuration suitable to generate a faster drive (rotational) speed. Testing has shown that by driving a centrifugal separator at a rotational speed closer to 10,000 RPMs, it is possible to demonstrate drastic soot reduction from an approximate 4.1 percent level to an approximate 0.8 percent level in the lubricant fluid in 280 hours of sump circulation (off-engine testing). The present invention provides an improved structure for a cone-stack centrifugal separator which is capable of generating the desired 10,000 RPM speed without needing to increase the lube system pressure above the normal and desired operating pressure of 70 PSI. The operating pressure range is from approximately 40 PSI to an upper limit of approximately 90 PSI.
One concern with this range of pressure is that the bearings which support the rotor need to be designed to withstand and contain the pressure inside the rotor. While journal bearings are preferred for these elevated pressure levels, these bearings have a rotational drag coefficient, caused by viscous shear of thin oil film between bearing and shaft, which precludes the cone-stack centrifuge from being driven at the desired 10,000 RPM (or higher) speed. By reducing the operating pressure inside the centrifuge rotor, roller bearings are able to be used which have a substantially lower drag coefficient, allowing a higher speed of rotation.